1. Field of the Invention
The invention relates to a current limiting device.
2. Description of the Related Art
Current limiting devices are used in a wide variety of applications to handle fault conditions when a current surges above a safe limit. In high current applications such as power supply lines and the like, it has been known in the past to pass the current through a coil which is provided about a leg of an iron former. Another leg of the former provides the core of a superconducting coil which is activated to hold the iron former in a saturated condition. Thus, under normal conditions, the iron is saturated and so effectively the coil carrying the current sees an air core. When a fault occurs, the current rises causing a consequent increase in the magnetic field generated by the coil which opposes the field due to the superconducting coil. This causes an increase in the permeability of the iron core and this increases the voltage across the coil carrying the current which limits the current being carried.
Although this current fault limiter is effective, it is very expensive due to the need to provide the iron core and the complexities due to the need to cool the superconducting coil to liquid helium temperatures.
More recently, it has been proposed to use a superconducting switch. In this case, a length of high temperature (HTc) superconductor is placed into the circuit carrying the current. HTc materials have a critical temperature which is relatively high (typically equivalent to a liquid nitrogen temperature) and have a critical current (strictly current density) which varies inversely with an applied magnetic field. If the current carried by the superconductor exceeds the critical current then the material of the conductor makes a transition to a resistive state which acts to limit the current being carried. The critical current value at which this transition occurs can be changed by changing the applied magnetic field.
In U.S. patent application Ser. No. 08/737,080 we describe a current limiting device which can be controlled to recover from a resistive state without terminating the flow of current.
In all these devices, in order to achieve the superconducting condition, the superconductor must be cooled to or below its critical temperature. Conventionally, this is achieved by immersing the superconductor in a boiling liquid coolant, or by passing such a coolant past the superconductor. In the case of high temperature superconductors, the liquid is typically nitrogen which boils at 77K.
One of the problems with this approach to cooling the superconductor is that the rate of heat transfer varies significantly depending upon the temperature difference between the liquid nitrogen and the superconductor. As can be seen in FIG. 1, when the superconductor has a temperature in the region of 77K, the boiling point of liquid nitrogen, the rate of heat transfer to the boiling liquid is high as indicated in a region 21 in FIG. 1. However, at higher temperatures as indicated in the region 22, the rate of heat transfer suddenly drops to much lower values as the cooled surface becomes occluded by a film of gas (Monroe et al, J. Applied Physics, p619 (1952)). This can lead to a risk of break down of the device since heat cannot be transferred away sufficiently quickly. The result is that the temperature of the superconductor must be very carefully controlled to stay within the region 21, which is undesirable.